The use of ultra filtration to concentrate a feed stream by passing smaller molecules through the filter, while retaining larger molecules are well known in the industry. Uses of ultrafilters are in particular widespread within the dairy industry and in the pharmaceutical industry. Another well-known use is known as reverse osmosis where essentially all molecules larger that water is retained and the permeate is pure water. Reverse osmosis is used e.g. for desalting seawater in order to produce sweet water for household use or irrigation.
The basis for all these uses is membranes having suitable permeability properties for the intended use. As the throughput obviously is dependent on the surface area of the membrane it is desirable to use large areas of membrane. In order to avoid voluminous process equipment, such membranes are often arranged in a spiral wound configuration.
Typical spiral wound filters consist of 1 to 6 spiral wound elements coupled in a serial flow mode and placed in a cylindrical pressure vessel. Typical spiral wound elements consist of one or more membranes of approximately 1×1 m, wound to a roll having a final diameter of 10-20 cm and a length of approximately 1 m. Between two membranes in the roll is placed a permeable porous medium for conduction of fluid, the concentrate spacer, to ensure that the concentrate can flow over the membrane in order to be distributed all over the surface and to continuously rinse the membrane from accumulating solids.
It is known in the area to provide the filter elements with a hard impermeable shell outside the wound filter element in order to keep the element tightly wound. In this configuration flow in and out of the filter element will be through the ends in an axial direction.
The flow inside the concentrate spacer may be in an axial or a non-axial direction, where the non-axial direction is in a spiralling tangential movement from the outside towards the centre of the wound spiralling element. It is known within the area that some designs of the concentrate spacer allow tangential spiralling movements whereas other designs do not.
Each membrane is typically composed of a porous central conducting medium, the permeate spacer, connected to a central permeate pipe, and on each side of the permeate spacer a separating membrane is provided. The assembly is blocked at the three edges not connected to the permeate pipe e.g. by glue, in order to secure that only fluid penetrating the separating membranes can enter into the permeate spacer.
Often a porous permeable tissue, the trim spacer, is wound around the spiral wound filtration element in order to minimize the space that inevitable occur between the spiral wound element and the pressure vessel.
At each end of the filter elements and in the interspace between two elements when two or more spiral wound elements are present in a cylindrical vessel anti telescoping devices (ATD) are usually provided, which serve as separators between two elements and to reduce the tendency of the spiral wound elements to unwind by telescoping. A number of different designs of ATDs are known within the area.
U.S. Pat. No. 4,296,951 discloses an spheroidal interconnector for filtration modules comprising an molded spheroidal body of elastomeric material having coaxial bores for receiving the respective ends of permeate tubes. These interconnectors are useful at various pressure ranges.
U.S. Pat. No. 4,301,013 discloses a spiral membrane module with controlled by-pass seal, where a material is provided in the space between the exterior surface of the filtration module and the interior surface of the cylindrical vessel in order to prevent accumulation of any product in this compartment.
U.S. Pat. No. 4,855,058 discloses a high recovery spiral wound membrane module comprising means for providing radial flow for the feed-concentrate mixture to an extend sufficient to achieve a conversation of 30% or greater while maintaining turbulent or chopped laminar flow.
U.S. Pat. No. 6,224,767 disclose a fluid separation element assembly where the anti telescoping devises are detachable making them reusable when the membrane elements has reached the end of their efficient life and have to be replaced by new elements.
In use the fluid to be concentrated is forced into the inlet of the pressure vessel and is pressed through the filter elements mainly in the axial direction, even though some filter elements also provide for some flow in the radial direction. However a part of the fluid will pass the filter element through the space between the filter element and the cylindrical vessel creating a by-pass flow.
The person skilled in the art will appreciate that the pressure drop along the filter element is dependent on the flow resistance encountered at the route the liquid travels. Therefore the pressure profile in the space between the filter element and the vessel will be different from the pressure profile at a path inside the spiral wound filter element even though the starting and final pressures are identical, i.e. at some locations the pressure is identical, at some locations the pressure is higher in the space between the filter element and the vessel and at some locations it is lower.
In the locations where the pressure is higher inside the filter element than in the space between the filter element and the vessel there is a tendency of the spiral wound element to unwind or to telescope with the result that channels are formed inside the filter element, which significantly reduces the efficiency of the filter element.
In practice it is experienced that the tendency to unwinding or telescoping increases with higher pressure gradients and flow velocity of the liquid with the consequence that the unwinding or telescoping effect limits the pressure gradient that can be applied to a spiral wound element, and because the pressure difference is the driving force in the filtration operation the efficiency of said filter element is limited.
It is desired to be able to operate spiral wound filter element at higher-pressure gradients in order to enhance the efficiency of the filter element.